The invention relates to an infrared objective.
The German patent publication 3,716,358 (corresponding to the European patent publication 0 290 751 A2) discloses an infrared radiation device which possesses a cooled Dewar vessel, which includes a sheet-like infrared receiver or detector element, preferably in the form of a focal plane array (FPA) and which is shut off from the infrared optical system by an infrared window transparent to infrared radiation. The Dewar vessel comprises a housing connected with a source of vacuum, and which is shut off from the outside by the infrared window. Inside the housing, opposite to the infrared window an internal cold shield is connected with a cooling device and comprises an internal cold diaphragm and whose cooled bottom wall serves as a support for the focal plane array. Optically in front of the Dewar vessel there is an external cold shield, which is thermally separated from the cooling device and which is so designed and so arranged in relation to the internal cold shield that the aperture of the internal cold diaphragm is projected onto itself by the external cold shield. In this arrangement the external cold shield is for example designed in the form of an internally coated concave mirror with an aperture constituting an external cold diaphragm, the center of curvature coinciding with the center of the internal cold diaphragm. This prior art however does not provide any complete teaching indicating simple means for the cooling effect to be provided for the internal cold shield so that less complicated equipment is required.
The known cold shield is referenced 1 in the diagrammatic FIG. 1a of the accompanying drawing omitting the Dewar vessel and the external cold shield. The cooled bottom wall 2 serving as a support for the detector element 3 is adjoined by side walls 4 and 5, which may be cooled. The cooling device is not illustrated either. At a distance z.sub.o from the image plane adjacent to the surface of the detector element 3 the internal cold diaphragm 6 is located having a central aperture 7. The cold diaphragm 6 adjoins the cooled side walls 4 and 5 and may also be cooled. The aperture 7 of the cold diaphragm possesses a predetermined marginal height h.sub.0 above the optic axis x. The cooled bottom wall 2 with the adjoining side walls 4 and 5 and the cold diaphragm 6 located between the same constitute the cooled cold shield 1 within a Dewar vessel, which is not illustrated and is connected with a vacuum device. The aperture ratio k.sub.0 of the cold diaphragm is defined as z.sub.0 /2h.sub.0, z.sub.0 being the distance of the aperture 7 of the cold diaphragm 6 from the image plane and h.sub.0 being the marginal height of the cold diaphragm. The image height y.sub.0 on the surface of the detector element 3 is .+-.d at the most. The internal cold diaphragm 6 in this respect constitutes the aperture diaphragm and constitutes the exit pupil of the infrared optical system, which is not illustrated. The external cold shield in accordance with the initially mentioned prior art is not illustrated in FIG. 1a either, since it has no effect on the spatial arrangement of the Dewar vessel and, respectively, of the internal cold shield.
Conventional sheet-like detector elements in known cold shields operate with a cold diaphragm aperture ratio z.sub.0 /2h.sub.0 of 1 to 3.
In order to be able to design the input infrared system with a minimum optical aperture (minimum optical complexity, minimum aberration) in the case of normal prior art arrangements (FIG. 1a) a relatively large distance z.sub.0 between the image plane on the surface of the sheet-like detector element 3 and the internal cold diaphragm 6 must be selected, such cold diaphragm 6 constituting the aperture diaphragm, and the exit pupil of the infrared optical system, for the shorter the distance z.sub.0 between the image plane on the surface of the sheet-like detector element 3 and the internal cold diaphragm 6 is, the larger the angle .theta..sub.d(max) between the ray sheaf of the marginal points of the detector element and the optic axis. This leads to disadvantages as regards weight, volume and price owing to the large lens diameters of the infrared optical system. In the case of a minimum optical aperture of the infrared optical system in practice with an optimum angle .theta..sub.opt the minimum distance between the image plane and the cold diaphragm is hence equal to about twice the diagonal diameter of the focal plane array or detector element, if vignetting of the optical system or a poor internal efficiency of the cold shield is to be avoided. In the case of modern large area infrared detector elements this leads to a correspondingly large volume of the internal cold shield or, respectively, to a correspondingly large Dewar vessel. The cooling effect to be provided by the cooling device connected with the internal cold shield is accordingly substantial.